The present invention relates to latex resins and, more particularly, to an improved method for shear coagulation of latex resins. The method is especially useful for shear coagulation of a latex of an acrylonitrile-butadiene-styrene, i.e., ABS, type resin.
Shear coagulation is a known step in recovering synthetic resinous solids from a latex obtained, for example, from conventional emulsion polymerization processes. Such processes are used in the manufacture of polymers such as rubber modifiers and rubber-modified polymers such as acrylonitrile-butadiene-styrene, acrylonitrile-methacrylate-butadiene-styrene, methacrylate-butadiene-styrene, toughened styrene/maleic anhydride, styrene maleimide and poly(methyl methacrylate). For example, a conventional emulsion polymerization process involves steps of preparing a rubber latex by emulsion polymerization, polymerizing monomers in the presence of the rubber latex and then recovering latex rubber solids. The emulsion polymerization method allows good control of particle size and size distribution, degree of grafting and cross-linking of the rubber, and generally, of the structure and properties of the final polymeric product.
Rubber particles of small particle size (less than 1,000 Angstrom (.ANG.) diameter) are easy and fast to produce and tend to provide formed articles having high gloss. But small size particles also tend to provide formed articles having reduced impact resistance. Thus, in many instances it is desirable to obtain compositions possessing a balance in properties by having a distribution of rubber particles ranging from relatively large to relatively small; i.e., a polydisperse particle size distribution. Rubber particle size and distribution can be controlled by adjusting surfactant concentration during polymerization or by a method of agglomerating larger rubber particles from smaller ones. For example, U.S. Pat. No. 3,032,524, May 1, 1962 to Brewer discloses a method of freeze agglomerating a latex after lowering the pH of the latex by injecting a stream of an acidic gas, such as carbon dioxide, therethrough. Other procedures to effect limited or controlled agglomeration are taught in U.S. Pat. No. 4,419,496, Dec. 6, 1983 to Henton, et al. and in the numerous references cited therein.
Recovery and subsequent processing of the rubber or rubber-modified particles from the latex is generally accomplished by coagulation of the latex; i.e., by destabilizing the colloidal dispersion of rubber or rubber-modified particles in the latex. For example, U.S. Pat. No. 4,302,377, Nov. 24, 1981 to Gurak, et al. discloses a process for coagulation of an aqueous latex by adding aqueous solutions of inorganic acids or inorganic electrolytes thereto. Another conventional method for coagulation of a latex is that of shear coagulation such as is taught in U.S. Pat. No. 4,299,952, Nov. 10, 1981 to Pingel, et al. and in U.S. Pat. No. 3,459,693, Aug. 5, 1959 to Halper, et al.
Shear coagulation is a highly advantageous method of coagulation since it does not introduce contaminants into the product or process water. However, efficient shear coagulation is difficult to obtain. For example, in a conventional process it is necessary to add sufficient soap to the reactor to almost entirely coat the latex particle surfaces in order to have a clean reactor after producing the latex. If an insufficient amount of soap is added to the reactor, there is an unacceptable build-up on the reactor walls. However, the amount of soap required to avoid build-up on the reactor walls provides a latex which is too stable for efficient shear coagulation.
The present invention overcomes this problem and provides an improved method for shear coagulating latices. The method is efficient, is usable with a wide variety of latices, and minimizes the levels of deleterious, contaminating components remaining in the resin or process water. Addition of calcium chloride, alum, sulfuric acid or other nonvolatile material is not required for coagulation in the present process. Thus, the rubber or rubber-modified product recovered in accordance with the present invention has good light color, i.e., is not yellow, since no contaminants are introduced during shear coagulation step. Further understanding of the present invention will be had from the following disclosure wherein all parts are by weight unless otherwise indicated.